The invention relates to a circuit for coupling a signal processing device to a signal transmission line. The circuit comprises a series connection of a first transistor and a controlled current source connected to a supply voltage source, the junction of the first transistor and current source being connected to the transmission line. The first transistor is driven by the output signals of the signal processing device and the current source is connected in during each period of transmission of the device.
A coupling circuit of this type is known, for example, from the German patent publication No. OS 32 15 063 (corresponding to European published patent application No. 093,882). The environment of such a circuit may include a transmission system wherein transmission is effected at low impedance in order to match a low impedance transmission path, while the receivers may have high impedance in order to load the transmission path as little as possible. This problem arises, in particular, when the electric power system installed in a building is employed as the transmission path for data. Such systems are utilized, for example, for the selective remote control of electrical devices located in buildings. In this case, encoded information in the form of address or command messages, transmitted as carrier frequency pulses (intermittent sine wave trains) are relayed via the electric power system. Due to the operation of the various electrical power devices, the installed electric power system is subject to strong impedance fluctuations. In order to substantially reduce the variations in the voltage level of the carrier frequency sine wave trains due to such impedance fluctuations, it has been suggested, according to German patent publication No. OS 32 15 063, to provide a coupling circuit which establishes low-impedance coupling for the duration of a command message but which is otherwise switchable with high impedance.